spi: sc18is602: Move checking chip_select for SC18IS602 to sc18is602_setup

So it will be checked when spi device is added onto the spi bus.
spi_add_device() calls spi_setup() which then calls spi->master->setup().
No need to check it every time sc18is602_transfer_one() is called.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@linaro.org>